In holographic storage systems data is stored as holograms resulting from the interference of a signal and reference beam. During storage, both the reference and signal beams are incident on the storage medium. During retrieval, the reference beam is incident on the medium and the signal beam is not generally incident on the medium. The reference beam diffracts off a stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages can be stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since a large number of bits within one page can be stored and retrieved simultaneously.
The implementation of holographic storage and retrieval techniques in a commercially viable system will benefit from simple and robust design of the relay optics. Additionally, various media types and geometries, e.g., holographic tape, disk or bulk holographic materials have to be supported by such a system.
One of the proposed approaches for simplifying the optics of a holographic system involves combining the signal and reference beams and passing them substantially along a common optical axis through shared optical elements. Such systems are suitable for the co-propagating geometries where the signal beam, reference beam and also the reconstructed beam all travel in the same general direction along a common optical axis. By virtue of this geometrical arrangement the co-propagating systems are well-suited for use with flat holographic media such as disks or tapes.
In U.S. Pat. No. 4,045,115, Lee describes an optics arrangement for a multi-track holographic tape recorder in which a reference beam may pass through the same series of lenses and directed to the same location in the holographic tape as the signal beam. In Lee's arrangement the signal and reference beams are co-propagating. In U.S. Pat. No. 3,947,640, Ruell et al. teach a method and device for recording data as holograms in a multi-channel storage tape also using a co-propagating geometry. The signal and reference beams are combined in a beam splitter and passed through a common lens directing the beams to the holographic tape medium. Another tape-based holographic system employing the co-propagating signal and reference beam geometry and a shared lens is described in U.S. Pat. No. 3,657,473 by Corcoran. Yet another information recording apparatus taking advantage of the same optics for directing the signal and reference beams to a holographic medium is described by Tatemichi et al. in U.S. Pat. No. 5,128,693. Still another system is taught in U.S. Pat. No. 3,573,362 by Burchardt. This invention describes a co-propagating holographic system where the signal and reference beams are combined and passed through a common mask before impinging on a holographic medium.
Although the above-mentioned holographic systems do provide shared optical elements for guiding the signal and reference beams they are not practical for use in a high-density page-based holographic system.
Pages of holographic data are generally produced with the aid of a spatial light modulator (SLM), which encodes the data in object pixels. For example, in a binary system a light pixel can represent a channel bit of value one and a dark pixels can represent a channel bit of value zero. It is important that the light from each object pixel be properly guided to the medium and, upon reconstruction projected onto the corresponding image pixel of a detection apparatus, e.g., a charge-coupled device (CCD). In other words, the luminous energy emanating from the object pixels has to be appropriately relayed by the optics of the holographic system to the corresponding image pixels. Meanwhile, the quality of transmission of the reference beam is not generally subject to the same quality conditions.
Hence, the prior art solutions do not provide for an effective optical relay for a high-density page-based digital holographic optical storage and retrieval system in a co-propagating geometry.